Method and apparatus for automatically focusing a microscope

ABSTRACT

A microscope system moves a target in a first direction relative to a low power objective lens and, during the relative motion, generates and records values of an electronic focus signal that depends on the magnitude of light reflected by the target. Then, a host workstation calculates a first estimate of position (&#34;focus position&#34;) of the target at which the microscope system is focused, by a median point method. In the median point method, the host workstation calculates the sum of the recorded values and determines the position along the range of motion at which half of this sum was exceeded, to be a first estimate of the focus position. From the intensity values of the first pass, optimal sensor gain is set for subsequent passes. Second and third estimates of the focus position can be calculated in a similar manner if necessary and the target is moved to the most recent estimate of the focus position. In one embodiment, the microscope system uses an area scan in which the largest value of an electronic focus signal at a set of points within an area of the target is recorded at a given elevation of the target. The largest of the recorded values is then used to estimate the focus position of the brightest layer of the target. In one embodiment, the microscope system focuses on a predetermined layer using an offset from the brightest layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 08/373,145,filed Jan. 17, 1995, now U.S. Pat. No. 5,672,861 which is acontinuation-in-part of Ser. No. 183,536 filed Jan. 18, 1994 now U.S.Pat. No. 5,483,055.

This application is related to and incorporates by reference, commonlyowned U.S. patent application Ser. No. 08/198,751 (Attorney Docket No.M-2465), filed Feb. 18, 1994, now U.S. Pat. No. 5,557,113, issued Sep.17, 1996, entitled "Surface Data Processor."

This application is also related to and incorporates by reference,commonly owned U.S. patent application Ser. No. 08/080,014, (AttorneyDocket No. M-2466) entitled "Laser Imaging System for Inspection andAnalysis of Sub-Micron Particles", filed by Bruce W. Worster et al, onJun. 17, 1993 now U.S. Pat. No. 5,479,252, issued Dec. 26, 1995.

CROSS-REFERENCE TO ATTACHED APPENDICES

Microfiche Appendices A-E (2 sheets of 158 frames) that are attachedherewith, are parts of the present disclosure and are incorporatedherein by reference in their entirety. Microfiche Appendices A and B area listing of routines that are used in microprocessors of coarse andfine Z controllers in two alternative embodiments of a microscopesystem. Microfiche Appendix C is a listing of routines that are used ina programmer to program a Programmable Logic Device (PLD). MicroficheAppendix D is a listing of components used in one embodiment of amicroscope system. Microfiche Appendix E is a listing of routines thatimplement computations for various auto-focus operations.

Appendix F is a listing of host workstation software that sets upvarious parameters for use in the area scan method. Appendix G, entitled"Ultrapointe Model 1000 Laser Imaging System Users' Manual" attachedherewith is part of present disclosure and is incorporated herein byreference in its entirety.

NOTICE OF COPYRIGHT RIGHTS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent files or records, but otherwise reserves all copyrightrights whatsoever.

FIELD OF THE INVENTION

The present invention relates generally to a method and apparatus forautomatic focusing. More particularly, the present invention relates toautomatic focusing of a confocal microscope.

BACKGROUND OF THE INVENTION

Certain prior art auto-focus microscopes generate an electronic focussignal, that is related to the distance of the target from an objectivelens of the microscope, to determine the position at which the target isin focus (henceforth "focus position").

In other auto-focus microscopes, a white light camera is used to detecta maximum contrast in the camera picture to indicate when the target isin focus. Measuring the picture contrast requires a large amount ofcomputer processing. This method is therefore slow and complicated.Additionally, this method fails if the target has no contrast (i.e., ifthe target is flat and has a uniform color, or is mirror-like).

The above method can fail depending on the amount of noise in theelectronic focus signal, such as the noise introduced by vibration ofthe target during relative movement between the target and themicroscope. Such noise can result in detection of a false focus positionwhen a microprocessor uses a low power objective lens. A low powerobjective lens is a lens that has a low power, such as 1.5X or 5X, asopposed to a high power objective lens that has a high power, such as100X or 200X.

Use of a low power objective lens can result in a large depth of focus,for example, greater than half the range of motion of the target withrespect to the microscope. A large depth of focus combined with noisecan result in an electronic focus signal that does not have a singledistinct well defined peak that is essential for proper operation ofcertain conventional microscopes. In such a case, a microscope can use afalse peak to move the target to a position at a distance from the focusposition, thereby requiring manual focusing.

In certain conventional microscopes, an electronic focus signal that isused for automatic focusing is generated from an illumination spot heldstationary on the target. The illumination spot could be broad bandwhite light or monochromatic laser light. Local height variations in thesurface of the target can cause the focused condition for the entirefiled of view to be less than optimal if the spot at which focusing isperformed is significantly higher or lower than the elevation of theremainder of the target's surface in the field of view. Therefore intargets, such as semiconductor substrates, that consist of highlyregular orthogonal lines, for example, row and column bit lines of aDRAM, the microscope can produce different results depending on whethera portion of a DRAM line or a portion of the substrate between two DRAMlines lies within the spot. If the spot is on a portion of the substratein a trench, the microscope focuses on the bottom of the trench. If thespot is on the top of a levee, the microscope focuses on the top of thelevee.

However, it is desirable that the microscope focus on the same layer allthe time, for example, on the top layer, regardless of what layerhappened to be within the spot, and regardless of the materials on thetop layer. In the multi-layer structure of a substrate, the differentlayers can be of different materials with different reflectivities. In atypical wafer, the top layer is a transparent passivation layer, and apredetermined layer that is typically desired is under the top layer.There is a need to focus on such a predetermined layer even if thepredetermined layer does not have the highest reflectivity.

Speed, accuracy and repeatability are additional desirablecharacteristics for focusing a microscope. The time required toautomatically focus a microscope determines the number of targets thatcan be viewed in a given time period, or, equivalently, determines theamount of time required to view a given number of targets, and therebydetermines the cost associated with viewing each sample.

Certain prior art microscopes utilize auto-focus optics that areseparate from the imaging optics of the microscope. In such microscopes,any "drift" between the auto-focus optics and the imaging optics resultsin loss of auto-focusing accuracy and repeatability.

It is therefore desirable to have a microscope that can automaticallyfocus on a predetermined layer of a target quickly, accurately andrepeatably.

SUMMARY OF THE INVENTION

In accordance with the present invention a confocal microscope system(henceforth "microscope system") uses a median point method for a coarseautofocus operation and an area scan method for a fine autofocusoperation to provide accurate, repeatable and high-speed automaticfocusing on any predetermined layer of a target. During an auto-focusoperation, such as a coarse auto-focus operation or a fine auto-focusoperation, the microscope system moves a target with respect to anobjective lens, while an electronic focus signal is measured. In acoarse auto-focus operation, the value of the electronic focus signal isrecorded periodically at large distances between a target's elevation,as compared to smaller distances used in a fine auto-focus operation.

The electronic focus signal generated by the microscope system is anovel signal that has a magnitude proportional to the amount of lightreflected by the target. The electronic focus signal reaches a maximum(sometimes referred to as "peak") when the target is in focus. Theelectronic focus signal is used to control an auto-focus operation inwhich the target is moved to its focus position (an elevation at whichthe target is in focus) with respect to the objective lens of a confocalmicroscope. Such an electronic focus signal provides a very preciseindication of the focus position of the target. In one embodiment, theelectronic focus signal is also used to generate an image of the target.Since the same signal (electronic focus signal) is used for bothfocusing and imaging, a good image signal is obtained after theauto-focus operation. The narrow peak also allows the microscope systemto discriminate between semi-transparent layers of the target.

In a coarse auto-focus operation, when using an objective lens havinglow power (e.g. 1.5x or 5x), the microscope system moves the target in astart-up move to a predetermined starting position so that the directionof the focus position is in a first predetermined direction (e.g.positive Z-axis direction) with respect to the starting position. Thenthe microscope system moves the target in a first pass (a movementduring which electronic focus signal values are recorded), in the firstdirection through a predetermined first distance (e.g. one-third of thecomplete range of motion) and after completion of the target's movementcalculates a first estimate of the focus position by the median pointmethod. In the median point method, a host workstation calculates thesum of the recorded values and determines the target's position at whichhalf of this sum was exceeded, to be a first estimate of the focusposition. After calculating an estimate of the focus position, themicroscope system moves the target to this focus position estimate.

Depending on the amount of jitter, or lack thereof in the movement ofthe target, additional or fewer estimates of the focus position can becalculated prior to movement of the target to the focus position. Thenumber of estimates are kept to a minimum, to get the highest speedpossible.

In one embodiment, after a first pass in a coarse auto-focus operation,the microscope system makes an optional second pass (similar to thefirst pass), to move the target through a predetermined second distancethat includes positions on both sides of the focus position's firstestimate. During the second pass, the microscope system again recordsthe values of the electronic focus signal, and calculates a secondestimate of the focus position, again using the median point method.Although the second pass is optional, a coarse auto-focus operation fora low power objective lens that includes two passes has the advantage ofaccurate and repeatable focusing even in the presence of noise thattypically causes conventional microscopes to malfunction. Also, such anauto-focus operation does not require a threshold and allows focus to befound quickly over a large range, but with high accuracy from the secondpass. Finally, the median point method is less susceptible to stagebacklash and stage motion irregularity than other conventional methods.

An auto-focus operation can be implemented with any one of three methodsfor generating the electronic focus signal: (a) spot method, (b) linescan method or (c) area scan method, in which a microscope systemmeasures the value of an electronic focus signal for each elevation ofthe target (a) as the illumination spot is held stationary, (b) as theillumination spot is scanned along a line, or (c) as the illuminationspot is scanned in an area in the field of view respectively. In a linescan method and an area scan method, the electronic focus signal can bemeasured at discrete spots or alternatively measured continuously overan infinite number of spots. In each of these three methods, themicroscope system moves the target through several elevations, anddetermines the elevation that generates the peak (e.g. largest value) ofthe electronic focus signal.

Automatic focusing can be based on a line scan, so that the electronicfocus signal is generated for several spots along a line segment on thetarget and the microscope automatically focuses on the brightest featureof the target that is in that line segment.

Automatic focusing can be based on an area scan, using an area peakdetector (sometimes referred to as "peak detector") that generates and amicroprocessor that records the largest value of the electronic focussignal as the illumination spot scans an area.

The microprocessor computes an estimate of the focus position by usingthe recorded values of the electronic focus signal for each targetelevation. In one embodiment, during a pass in a fine auto-focusoperation, the microprocessor estimates the focus position to be thetarget elevation at which the largest recorded value (sometimes referredto as "peak value") of the electronic focus signal was generated, andthe microscope system moves the target to focus on the layer ("brightestlayer") that generated this largest recorded value.

A predetermined layer that the user is interested in focusing on can bereached using an offset from the brightest layer. Use of the brightestlayer and the offset results in an accurate and repeatable auto-focusoperation that is dependent only on the brightest layer's reflectivityand is not influenced by the shape, location, or extent of otherfeatures on a target. Such an auto-focus operation is also not dependenton prior knowledge of the reflectivity of the predetermined layer.

The invention will be more fully understood in light of the followingdrawings taken together with the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram that illustrates a confocalmicroscope system according to an embodiment of the invention.

FIGS. 2A-2C show a target of a confocal microscope below the focusposition, at the focus position and above the focus position,respectively, illustrating, at each position, the pattern of lightreflected from the target.

FIG. 3A illustrates the relation between the magnitude of an idealizedelectronic focus signal of a confocal microscope and the position of atarget with respect to an objective lens as the target is moved alongthe Z-axis.

FIG. 3B illustrates an idealized electronic focus signal after theauto-focus system has been initialized.

FIG. 3C illustrates an actual electronic focus signal for a microscopesystem that uses a low power objective lens.

FIG. 4 is a block diagram of one embodiment of a Z-axis controller usedto control a fine Z-stage and to provide feedback to a coarse Z-stage.

FIGS. 5A-5E are schematic diagrams of the embodiment of the Z-axiscontroller of FIG. 4.

FIG. 6 is a graphic representation of three passes performed during acoarse auto-focus operation according to the invention.

FIG. 7 is a graphic representation illustrating one embodiment of a fineauto-focus operation.

FIG. 8A is a graphic representation illustrating another embodiment of afine auto-focus operation.

FIGS. 8B and 8C illustrate the movement of a target along the Z axis ofFIG. 8A with respect to time in two alternative embodiments.

FIG. 9 is an isometric view of a fine Z-stage,

FIG. 10 is a cross sectional view of the fine Z-stage of FIG. 9.

FIG. 11 is an exploded isometric view of a piezoelectric element, sensorand bottom plate of the fine Z-stage of FIG. 9.

FIG. 12 is a block diagram of another embodiment of a Z-axis controllerused to control a fine Z-stage and to provide feedback to a coarseZ-stage.

FIGS. 13A, 13B and 13C are timing diagrams illustrating various signalsin one embodiment of a confocal microscope system.

FIGS. 14-21 are schematic diagrams of the embodiment of the Z-axiscontroller of FIG. 12.

DETAILED DESCRIPTION

FIG. 1 is a simplified block diagram of a confocal microscope system 100described briefly below and described in more detail in commonly owned,U.S. patent application, Ser. No. 08/080,014, now U.S. Pat. No.5,479,252, issued Dec. 26, 1995 entitled "Laser Imaging System forInspection and Analysis of Sub-Micron Particles", filed by Bruce W.Worster et al, on Jun. 17, 1993, that is incorporated herein byreference in its entirety.

Laser 102 (FIG. 1) generates a laser beam 123I that is transmittedthrough a polarizing beam splitter 104, a converging lens 130, a spatialfilter 131 (such as a pinhole), a collimator lens 132, a quarterwaveplate 133, reflected from an X-mirror 106 and a Y-mirror 108, andtransmitted through an objective lens 110 to the surface of a target112. In one embodiment, laser 102 is a conventional argon-ion laser,however, other types of lasers may be utilized in alternate embodiments.Target 112 is an object, such as a semiconductor wafer, that is to beviewed using microscope system 100. X-mirror 106 and Y-mirror 108 areeach rotatable about an axis such that the illumination spot created byincident laser beam 123I can be moved along an X-axis and a Y-axis,respectively, of target 112. Laser 102, beam splitter 104, converginglens 130, spatial filter 131, collimator lens 132, quarter waveplate133, X-mirror 106, Y-mirror 108 and objective lens 110 are eachconventional structures that are well known by those skilled in the artof confocal microscopes.

Laser beam 123I reflects from the surface of target 112 in a pattern(illustrated in FIGS. 2A-2C) that is dependent upon the distance ofobjective lens 110 from target 112. FIGS. 2A-2C show target 112 belowfocus position 203, at focus position 203 and above focus position 203,respectively. When target 112 is positioned below or above focusposition 203, respectively, a small percentage of light 123R fromincident laser beam 123I that is originally transmitted throughobjective lens 110 is reflected back through objective lens 110 in acoherent manner. The amount of reflected light 123R is illustrated bythe graph of the electronic focus signal as a function of position inFIG. 3A. There is a gradual transition in the amount of reflected light(depending on the target's position) between FIGS. 2A-2C. As is wellknown in microscope engineering art, the amount of reflected light alsodepends on, for example, reflectivity of target, angle of target andangle of the chuck that supports the target.

Microscope system 100 includes a spatial filter 131 with a pin hole thatpermits passage of incident laser beam 123I and some portion ofreflected light 123R, the portion depending on the position of target112. Spatial filter 131 blocks off the rest of reflected light 123R asillustrated in FIGS. 2A and 2C. However, when target 112 is positionedat focus position 203 (FIG. 2B), a maximum amount of light from incidentlaser beam 123I is reflected (light 123R) and transmitted back throughobjective lens 110 and the pinhole along a conjugate path of microscopesystem 100.

Reflected light 123R passes through objective lens 110, is reflected byY-mirror 108 (FIG. 1), X-mirror 106 and reflects off beam splitter 104to a photodetector 114. Photodetector 114 is a device, such as aphoto-multiplier tube (PMT) or photo-diode, that generates an electronicfocus signal 115 that is an analog signal that has a magnitude (voltagein one embodiment) proportional to the intensity of reflected light 123Ras measured by photodetector 114. The photodetector's gain is adjustableto accommodate different laser power, laser wavelength and targetreflectivity. As is well known in microscope engineering, when thetarget is in focus, the value of the detected electronic focus signal isproportional to: PMT's GAIN * TARGET'S REFLECTIVITY * LASER'S POWER. ThePMT's gain is adjusted empirically, as discussed below.

Electronic focus signal 115 (FIG. 1) is provided to host workstation 116and to Z-axis controller 118. Z-axis controller 118 is directly coupledto fine Z-stage 120 and is indirectly coupled to coarse Z-stage 122through host workstation 116 and coarse Z-axis controller 117. CoarseZ-stage 122 uses a motor, such as a stepper motor, to move target 112relative to objective lens 110 through a relatively large range ofmotion (e.g. a large percentage of total possible movement) along theZ-axis, in an operation referred to as "coarse auto-focus operation". Inone embodiment, coarse Z-stage 122 moves target 112 through 1000 micronsin a start-up move of a coarse auto-focus operation to a startingposition 516 (FIG. 6) from an initial position of target prior to theauto-focus operation (the total possible movement being approximately6000 μm).

In one embodiment of the present invention, coarse Z-axis controller 117is a conventional stepper motor controller available as part number310MX3 from New England Affiliated Technology (NEAT) of 620 Essex St.,Lawrence, Mass. 01841. Coarse Z-stage 122 can be driven by aconventional stepper motor (not shown) such as the Vexta C5858-9012available from Oriental Motor of 16-17 Veno 6-Chome Taito-Ku, Tokyo,Japan. In this embodiment, coarse Z-stage 122 is Part #1930237 availablefrom NEAT (above).

As explained in more detail below, fine Z-stage 120 uses apiezoelectrically driven element to move target 112 through a relativelysmall range of motion (e.g. a small percentage of total possiblemovement) along the Z-axis, in an operation referred to as "fineauto-focus operation." In one embodiment, fine Z-stage. 120 moves target112 through 50 microns in a fine auto-focus operation (the totalpossible movement being approximately 6000 μm). Although the inventionis described as having a movable target 112 and a stationary objectivelens 110, target 112 can be held stationary while objective lens 110 ismoved.

FIG. 3A is an idealized graph of the magnitude (sometimes referred to as"strength" or "intensity") of an electronic focus signal 115A as afunction of the position, e.g. elevation of target 112 along the Z-axis.Electronic focus signal 115A has a magnitude that is theoreticallyproportional to a sinc-squared function ((sin(x)/x)²), having a fullwidth half max measurement 305A that varies based on the numericalaperture of objective lens 110 and the wavelength of laser beam 123. Thefull width half max measurement 305A is the width, along the Z-axis,between two points 308A and 308B at which the magnitude of electronicfocus signal 115A is at half of its maximum magnitude 301A at focusposition 203. As illustrated in FIG. 3A, electronic focus signal 115Ahas a value of (A1+A2)/2 for the full width half max measurement 305A,wherein A1 is the background value (described below) and A2 is themaximum value for focus position 203. The relationship of the numericalaperture and the wavelength to the magnitude of the electronic focussignal is well known to a person skilled in the art of confocalmicroscopes.

In FIG. 3A, focus position 203 at the center of main lobe 307A is adistinct position, as shown by a sharp, well defined peak 301A in themagnitude of electronic focus signal 115A. Electronic focus signal 115Aalso exhibits side lobes such as lobes 306a-306b. Depth of focus(sometimes referred to as "width of focus") 302A spans a range along theZ-axis in which the magnitude of electronic focus signal 115 is greaterthan a background value 303. Background value 303 is a small value ofelectronic focus signal 115A that is non-zero and results from leakagecurrents and a small amount of background light that reachesphotodetector 114.

Depth of focus 302A becomes smaller as the numerical aperture ofobjective lens 110 increases or as the wavelength of laser beam 123decreases, as is well known. In the following discussion, objective lens110 has a power of 100X and a numerical aperture of 0.95, and laser beam123 has a wavelength of 488 nm in one embodiment. In this embodiment,electronic focus signal 115A has a full width half max measurement 305Aof approximately 0.5 microns.

Microscope system 100 automatically moves target 112 to an estimate offocus position 203 that results from one or more auto-focus operationsas described below. To initialize microscope system 100 for a coarseauto-focus operation, the gain of photodetector 114 is increased to anempirically predetermined autofocus gain. Also, an empiricallypredetermined zero position offset 310 is applied to electronic focussignal 115 such that background value 303 and side lobes 306a-306b areeffectively eliminated from electronic focus signal 115A. FIG. 3Billustrates electronic focus signal 115A after microscope system 100 hasbeen initialized.

The auto-focus gain of photodetector 114 (FIG. 1) and zero positionoffset 310 (FIG. 3B) are empirically determined by performing a seriesof auto focus operations using different targets 112 to cover as wide arange of targets as typically used by the user (e.g. an aluminum targetand a semiconductor target). Zero position offset 310 is selected toensure that background value 303 and side lobes 306a-306b are below zeroposition 320. A small positive value greater than zero position 320 isthen selected for use as threshold value 304. Once threshold value 304has been selected, the threshold value 304 can be used for subsequentauto-focus operations on a wide variety of targets.

When the magnitude of electronic focus signal 115A exceeds apredetermined threshold value 304, a near-focused condition is said toexist. During a near-focused condition, target 112 is relatively closeto focus position 203 (illustrated by positions 308a and 308b of FIG.3A). The gain of photodetector 114 and the zero position offset 310 arethen maintained during the coarse auto-focus operation.

Certain photodetectors 114, particularly photo multiplier tubes (PMTs),can be damaged when exposed to high signals (laser signals having amagnitude greater than magnitude that PMTs are designed for) that resultfrom increasing the gain of photodetector 114. Such damage is not aproblem in the current invention because PMT control circuitry has anoverload sensing circuit that automatically reduces PMT's gain to zeroby a conventional method, before damage occurs.

FIG. 3C illustrates variations in magnitude of an actual electronicfocus signal 115C. Magnitude of electronic focus signal 115C isirregular due to noise caused by, for example, vibration between target112 and objective lens 110 during movement of target 112. In FIG. 3C,objective lens 110 has low power (for example magnification of 1.5X or5X). The low power of objective lens 110 results in a large depth offocus, such as distance 305C (e.g. 30-100 microns) between elevations335A and 335B for the full width half max measurement. The large depthof focus and the irregular waveform of electronic focus signal 115C canresult in a false focus position, such as one of position 333A or 333B,if peak detection is used to estimate focus position 203.

Use of a threshold, as described above in reference to FIGS. 3A and 3B,when electronic focus signal 115C is generated by a low power objectivelens has the disadvantage that a small change in threshold, (e.g. from0.5 to 0.6) results in a large change in the target's elevation alongthe Z axis (e.g. from points 335A and 335B to points 336A and 336B) atwhich the threshold is exceeded. In such a case, focus position 203 ofidealized main lobe 307 is estimated in one embodiment without using athreshold, by summing up the magnitude of electronic focus signal 115Cat several elevations of target 112 in the range of motion along theZ-axis, in a median point method that is described below.

FIG. 4 is a block diagram of Z-axis controller 118 that controls fineZ-stage 120 and also provides feedback used to control coarse Z-stage122. Operation of some structures in FIG. 4 is described below, whileoperation of the rest is described in reference to FIGS. 5A-5E and 6-8.

Within Z-axis controller 118 (FIG. 4), electronic focus signal 115 istransmitted to a first input terminal of comparator 401 and to an inputterminal of an analog to digital converter (ADC) 407. ADC 407 measuresthe magnitude of electronic focus signal 115 when commanded bymicroprocessor 403, for example eighty (80) times a second. A digitaloutput signal (on 8 lines in one embodiment) of ADC 407 drivesmicroprocessor 403. Comparator 401 sets a flip-flop 402 when themagnitude of electronic focus signal 115 exceeds a threshold signal atthe comparator's second input terminal that is driven by a digital toanalog converter (DAC) 404. DAC 404 in turn receives a threshold valuefrom microprocessor 403.

The output terminal of comparator 401 (FIG. 4) is coupled to the setterminal of latching flip flop 402. The Q output terminal of flip-flop402 is coupled to an input terminal of status register 405.

An output terminal of control register 406 is coupled to the resetterminal of flip flop 402. Control register 406 resets flip-flop 402 oncommand from microprocessor 403.

Microprocessor 403 has terminals coupled to status register 405, controlregister 406, ADC 407 and host work station 116. Microprocessor 403 isalso coupled to position control register 408. The output of positioncontrol register 408 is transmitted through DAC 409, summing node 410,integrator 420, and amplifier 411 to provide a control voltage signal toa piezoelectric element 1130 of fine Z-stage 120. Summing node 410 alsoreceives a feedback signal (on line 435) that is linearly proportionalto the position of target 112 along the Z-axis i.e. distance from aproximity sensor 1135 of fine Z-stage 120. The feedback signal is usedto linearize the behavior of a piezoelectric element 1130 (FIG. 11).

FIGS. 5A-5E are schematic diagrams of one embodiment of Z-axiscontroller 118 of FIG. 4. Similar elements in FIGS. 4 and 5A-5E arelabelled with the same number. Ratings of various components illustratedin FIGS. 5A-5E are listed in microfiche appendix D (at pages 59-64) forone embodiment.

Central processing unit (CPU) 5000 (FIG. 5D) of microprocessor 403transmits and receives information through bus transceiver 5034 to 8-bitdata bus 5006. CPU 5000 is, for example, a TP-RS485 twisted pair controlmodule, model number 55050-00, available from Echelon of 4015 MirandaAvenue, Palo Alto, Calif. 94304. Bus transceiver 5034 providesadditional drive capability to CPU 5000. Bus transceiver 5034 is a wellknown device, available for example as part number 74ALS245, from TexasInstruments (TI) of 7839 Churchill Way, Dallas, Tex. 75251.

Address register 5036 receives addressing information from CPU 5000through data bus 5006. This addressing information determines theregister or device within Z-axis controller 118 that the microprocessor403 accesses. Address register 5036 is available, for example, from TI(above) as part number 74ALS573.

The output of address register 5036 is provided to address decoders 5038and 5040. Address decoders 5038 and 5040 decode the addressinginformation and generate signals that enable a register or device withinZ-axis controller 118 that microprocessor 403 accesses. Address decoders5038 and 5040 are available for example from TI (above) as part numbers74ALS138 and 74LS139, respectively. Microprocessor 403 communicates withposition control register 408, status register 405, control register406, DAC 404 and ADC 407 using 8-bit data bus 5006.

Registers 5001 and 5003 (FIG. 5A) within position control register 408receive positioning information from microprocessor 403 on data bus5006. Registers 5001-5004 are well known in the art of designingmicroprocessor based systems. Registers 5001 and 5003 are available forexample from TI (above) as part number 74ALS574. Registers 5002 and 5004are available for example from TI (above) as part number 74ALS273. Eightbit words on data bus 5006 are transmitted through registers 5001-5005of position control register 408 to provide a 12-bit input to 12-bit DACunit 5008 of DAC 409.

DAC unit 5008 is a conventional digital to analog converter (DAC), knownin the electronics art, and available for example from Analog Devices(AD) of 1 Technology Way, Norwood, Mass. 02062, as part number AD7541.The remaining ancillary elements of DAC 409 including operationalamplifier 5009 and the illustrated resistors, capacitors and diodes areconventional elements commonly seen in conventional circuits using DAC409, as is well known to a person skilled in the art of electronicengineering. Operational amplifier 5009 is available for example from AD(above) as part number OP-177E. DAC 409 provides an analog output signalon lead 5010.

As shown in FIG. 5B, lead 5010 is connected to one input terminal ofoperational amplifier 5012 of summing node 410. Operational amplifier5012 is available for example as part number AD712 from AD (above). Thesignal on the other input terminal of operational amplifier 5012 isderived from the position feedback signal (on line 435 in FIG. 4) ofsensor 1135 (FIG. 11) in fine Z-stage 120. Sensor 1135 provides an inputsignal to operational amplifier 5018. Operational amplifier 5018 isavailable for example from AD (above) as part number AD712.

Various illustrated resistors and capacitors, such as R7, C8, and R8,that are coupled to operational amplifier 5018 create a conventionalbuffer. Components rated as DNL (Do Not Load) in FIGS. 5A-5E and FIGS.14-21 are not used (i.e. not part of the circuit). The output signal ofoperational amplifier 5018 is provided to the other input terminal ofoperational amplifier 5012. The output terminal of summing node 410 iscoupled to the input terminal of integrator 420. Integrator 420 includesan operational amplifier such as part number AD712, available from AD(above). The associated resistors, capacitors and diodes such as R19,C20, R20, R21, C25, D5 of integrator 420 are known to one skilled in theart of designing electronics.

The output signal of integrator 420 is provided to notch filter 5013,which includes two resistors (e.g. R24, R23) and two capacitors (e.g.C22, C27). The output signal of notch filter 5013 is provided tooperational amplifier 5014, that is available, for example, from AD(above) as part number AD712. The output signal of operational amplifier5014 is provided to the input terminal of amplifier 411.

Amplifier 411 is a conventional amplifier that includes an operationalamplifier available from for example Apex Microtechnology of 5980 N.Shannon Rd., Tucson, Ariz. 85741 as part number PA85. The illustrateddiodes, resistors and capacitors such as R22, D3, D4, D8, C34, R14, C14,R5, R6, C7, R2, C1, R4 of amplifier 411 are all known in the electronicsart. The output signal of amplifier 411 is provided to piezoelectricelement 1130 within fine Z-stage 120.

As shown in FIG. 5C, electronic focus signal 115 is provided to ADC 407.Electronic focus signal 115 is routed through multiplexer 5022 tooperational amplifier 5020. Multiplexer 5022 is a conventional partavailable, for example from Siliconix of 2201 Laurelwood Road, SantaClara, Calif. 95058 as part number DG 211. Operational amplifier 5020 isavailable from for example AD (above) as part number AD843. Operationalamplifier 5020 buffers electronic focus signal 115. The output ofoperational amplifier 5020 is provided to an input of ADC unit 5021. ADCunit 5021 is a conventional part available for example as part numberAD7575 from AD (above). The other devices C17, C18, D2, R26, C23, C51,C52, U27 coupled to ADC unit 5021, as illustrated in FIG. 5C, are knownin the art of electronics. In response to electronic focus signal 115,ADC unit 5021 supplies an 8-bit digital signal representative ofelectronic focus signal 115. The 8-bit digital signal of ADC unit 5021is provided to microprocessor 403 on data bus 5006.

FIG. 5C also illustrates flip flop 402. Flip flop 402 is programmed asone of the devices present within programmable logic device (PLD) 5023.PLD 5023 is available for example from Lattice Semiconductor of 5555 NEMoore Ct., Hillsboro, Oreg. 97124, as part number GAL20RA10. The inputsignals to PLD 5023 include: a set input signal from comparator 401 anda reset input signal from control register 406. PLD 5023 processes theseinput signals and generates a Q output signal representing the outputsignal at Q output terminal of flip flop 402. This Q output signal isprovided to status register 405. (PLD 5023 also has input signals andoutput signals unrelated to auto-focus operations.) One embodiment ofinstructions to a programmer for programming PLD 5023 are listed in amicrofiche appendix C that is incorporated herein by reference in itsentirety. Instructions in microfiche appendix C can instruct programmermodel no. BP-1200 available from BP Microsystems, Inc. of 1000 N. PostOak Road, Houston, Tex. 77055.

FIG. 5D illustrates status register 405. Status register 405 is aconventional register available for example as part number 74ALS541 fromTI (above). As previously discussed, status register 405 receives the Qoutput signal of flip flop 402 from PLD 5023 (status register 405 alsoreceives other information unrelated to auto-focus operations). The8-bit output signal of status register 405 is provided to data bus 5006such that microprocessor 403 can detect when flip flop 402 is set.

Control register 406 (FIG. 5D) receives an 8-bit input signal frommicroprocessor 403 on data bus 5006. Control register 406 is availablefor example from TI (above) as part number 74ALS273. An output terminalof control register 406 is coupled to PLD 5023, such that a signal fromcontrol register 406 can reset flip flop 402.

DAC 404 also receives an 8-bit input signal from microprocessor 403 ondata bus 5006. This 8-bit input signal is transmitted through register5007 (available for example from TI as part number 74ALS574) toconventional DAC unit 5011 (available for example from NationalSemiconductor as part number DAC0808). DAC unit 5011 converts theincoming 8-bit signal into an analog output signal. This analog outputsignal is provided to an input terminal of operational amplifier 5017(available for example from AD as part number AD712). The output signalof operational amplifier 5017 is provided to an input terminal ofcomparator 401. Electronic focus signal 115 is provided to the otherinput terminal of comparator 401. Comparator 401 includes comparatorunit 5019, available for example from National Semiconductor of 2900Semiconductor Drive, Santa Clara, Calif. 95062, as part number LM311.The output signal of comparator 401 is provided to flip flop 402.

FIG. 5E illustrates conventional structures used in the power supplyconnections 5041, 5042 that supply power to fine Z-stage controller 118.Analog/digital grounding structure 5043 is used to connect analog anddigital grounds on fine Z-stage controller 118.

In one embodiment of the present invention, a coarse auto-focusoperation is performed as follows. Host work station 116 (FIG. 1),through coarse Z-stage controller 117, instructs coarse Z-stage 122 tomove target 112 in a start-up move (not shown) downward through astartup predetermined distance (e.g. 1000 microns) along the Z-axis fromthe current position of target 112. If there is less distance from thestartup predetermined distance between the current position and alowermost position (not shown) along the Z-axis, then target 112 ismoved to the lowermost position along the Z-axis. The lowermost positionis that position along the Z-axis below which target 112 cannot be movedby any mechanism in microscope system 100. This startup pass ensuresthat target 112 is positioned at a starting position 516 below focusposition 203. For example, as all focus positions are within a limiteddistance (≈200 μm in one embodiment) of an upper travel limit, aposition e.g. 1000 μm below any initial position (not shown) guaranteesthat starting position 516 is below focus position 203. Travel limitsare two positions of a target that are farthest from each other, such asa lower most position (not shown) and first safe operating position 520of FIG. 6. If the first time a target is loaded, coarse Z stage 122 isat its lowermost position, there is no startup pass. A startup pass alsoensures that target 112 is positioned at least a minimum distance belowfocus position 203, thereby allowing coarse Z-stage 122 to achieveconsistent start-up characteristics (i.e., velocity, acceleration, etc.)before target 112 encounters a focused condition.

Host work station 116 instructs microprocessor 403 to send a digitalsignal to DAC 404 such that the output signal of DAC 404 has a voltagelevel corresponding to threshold value 304 that was determined duringinitialization of microscope system 100. In one embodiment, the inputsignal to DAC 404 is a fixed digital input signal having a value of 24(out of a range of values between 0 and 255). In response to anothersignal generated by host workstation 116, control register 406 transmitsa reset signal to reset flip-flop 402 to its initial state (e.g., alogic "0").

FIG. 6 is a graphic representation of three coarse passes 513, 514 and515 that are performed during a coarse auto-focus operation according toone embodiment of the invention. A number of coarse passes other thanthree can be used in other embodiments. The vertical axis in FIG. 6illustrates the position (e.g. elevation) of target 112 (FIG. 1) alongthe Z-axis. The horizontal axis in FIG. 6 illustrates the magnitude ofthe electronic focus signal 115.

To perform a first coarse pass, host work station 116 (FIG. 1) instructscoarse Z-stage controller 117 to move (illustrated by arrow 512) coarseZ-stage 122, and thereby target 112, from starting position 516 inpositive Z direction 533 through a first safe operating distance 512 toa first safe operating position 520. If no focus condition is detectedduring a first coarse pass, target 112 is moved to first safe operatingposition 520. If a focus position is detected, the movement of target112 is illustrated by the three coarse passes, 513, 514 and 515.

First safe operating distance 512 is selected so that there is no chancethat target 112 will contact objective lens 110 that is located atposition 517. Any amount of clearance between first safe operatingposition 520 and position 517 can be selected. In one embodiment, thefocus position of each objective lens is measured and travel limits areprogrammed (e.g. soft coded into a software configuration table) so thatno contact occurs between target 112 and objective lens 110. All entriesin a configuration table are defined by a user during installation ofmicroscope system 100. In particular, travel limits for movement of thetarget are set by manual focusing on a target. In one embodiment, firstsafe operating position 520 is empirically set at 50 microns above focusposition 203 (i.e., 50 microns above the focal point of objective lens110) based on various parameters such as thickness of target and theworking distance (e.g. 270 microns) as described below. In oneembodiment, first safe operating distance 512, is selected from therange of 1000-6500 microns, depending on the initial position of target112, i.e., before target 112 is moved to starting position 516.

The working distance of objective lens 110 is the distance from theobjective len's position 517 (FIG. 6) to focus position 203. In variousembodiments, the working distance varies from 270 microns to severalmillimeters, depending on the numerical aperture of objective lens 110.

Safe operating position 520 depends on many factors, such as flatness ofthe target, flatness of the XY stage on which the target is supported,position of fine Z stage 120, thickness of the target and repeatabilityof a "home" position of coarse Z stage 122. Safe operating position 520is empirically selected to be above a typical focus position 203 for atypical target, but below the position at which the target touchesobjective lens 110. Safe operating position 520 is selected to besufficiently away from focus position 203 to ensure enough travel toobtain a well defined peak in the magnitude of electronic focus signal115.

In a threshold method, during first coarse pass 513, comparator 401continuously compares incoming electronic focus signal 115 withthreshold value 304 received from DAC 404. Because target 112 isinitially out of focus, electronic focus signal 115 is initially lessthan threshold value 304. Under these conditions, the output signal ofcomparator 401 has a positive voltage. As target 112 approaches focusposition 203, the magnitude of electronic focus signal 115 increases.When the magnitude of electronic focus signal 115 exceeds thresholdvalue 304, the output signal of comparator 401 transitions to a negativevoltage, thereby setting flip flop 402.

Once flip flop 402 (FIG. 4) is set, the voltage at the Q output terminaltransitions to a logic high state in this embodiment. Such a logic highstate at the Q output is transmitted to status register 405, causing abit within status register 405 to change value. Microprocessor 403,which continuously monitors status register 405, thereby detects thatflip flop 402 has latched. Upon detecting this latched condition,microprocessor 403 signals host work station 116. In response, host workstation 116 instructs coarse Z-axis controller 117 to stop coarseZ-stage 122, and thereby stop movement of target 112.

Because electronic focus signal 115 is an analog signal and flip flop402 latches when the magnitude of electronic focus signal 115 exceedsthreshold value 304, the possibility of missing a focused condition iseliminated if the peak magnitude is greater than the threshold value.There are no discrete sampling periods during which the focusedcondition may be missed. Thus, flip flop 402 latches even for a focussignal 115 having a narrow depth of focus 302, as illustrated in FIG.3A. Consequently, target 112 can be moved upward at a much fastervelocity than was possible with auto-focus microscopes of the prior art.

In one embodiment, the average velocity of target 112 during firstcoarse pass 513 is dependent on first safe operating distance 512.Target 112 is moved at an average velocity that enables target 112 tomove through first safe operating distance 512 in approximately onesecond. However, the maximum average velocity is approximately 3000microns per second in this embodiment. This allows first coarse pass 513to be completed in approximately 1 second.

If a focus condition is not detected before target 112 completesmovement through first safe operating distance 512, host work station116 instructs coarse Z-stage controller 117 to stop target 112 whenfirst safe operating position 520 is reached. A focus position canremain undetected depending on the parameters used to detect focusposition 203. For example, when a threshold value is larger than thepeak magnitude of electronic focus signal 115, the output of comparator401 does not transition to a negative voltage even as target 112 passesthrough focus position 203.

Because of the high velocity (e.g. 3000 microns per second) at whichtarget 112 is moved during first coarse pass 513 and the significantamount of time (e.g. 20-60 milliseconds) required to stop the movementof target 112 after a focused condition is detected, target 112 moves("target overshoot") to first stopping position 505 (FIG. 6) above focusposition 203 at the time that target 112 comes to rest. This targetovershoot is illustrated in FIG. 6, which shows that a focused conditionis detected at first trip position 504, and that target 112 comes torest at first stopping position SOS. Thus, it is necessary to performone or more additional passes to position target 112 closer to focusposition 203.

The distance required to stop target 112 during first coarse pass 513 isdependent upon threshold value 304 (because a lower threshold value 304causes flip flop 402 to latch sooner), the system gain (because a highersystem gain allows for earlier detection of a focused condition), thetime required for microprocessor 403 to detect that flip flop 402 haslatched, the time required for microprocessor 403 to communicate thisinformation to host work station 116, the time required for host workstation 116 to issue a command to stop coarse Z-stage 122, and thevelocity of coarse Z-stage 122 at first trip position 504. Suchdependencies are well known to a person skilled in engineering.

Host work station 116, through microprocessor 403, then instructscontrol register 406 to transmit another reset signal to clear flip flop402 (FIG. 4) before second coarse pass 514 is begun. This reset signalallows flip flop 402 to detect the next time electronic focus signal 115exceeds threshold value 304.

The steps performed during second coarse pass 514 are similar oridentical to those of first coarse pass 513. Host work station 116instructs coarse Z-stage 122 (through coarse Z-stage controller 117) tomove target 112 in negative Z direction 532 at a velocity that is slowerthan the velocity of target 112 during first coarse pass 513. Thismotion continues until a near-focused condition is detected at secondtrip position 506, causing flip flop 402 to latch, or until target 112moves through a predetermined second safe operating distance 507 tosecond safe operating position 522. This second safe operating distance507 is selected to assure that target 112 travels through focus position203, based on the width of electronic focus signal 115 and the range ofanticipated overshoot of target 112 during first coarse pass 513. Secondoperating distance 507 is selected greater than the overshoot from firstcoarse pass 513, plus the width of electronic focus signal 115.

In one embodiment, second safe operating distance 507 is 1200 microns,unless target 112 was stopped less than 50 microns from first safeoperating position 520. If target 112 was stopped less than 50 micronsfrom first safe operating position 520, the second safe operatingdistance 507 is smaller, e.g. 200 microns in this embodiment. The lattersecond safe operating distance is smaller because target 112 isdecelerating near the end of first safe operating distance 512 so thatit is easier to stop target 112, thereby decreasing the amount of targetovershoot so that target 112 is stopped closer to focus position 203than would otherwise be the case. Because target 112 is closer to focusposition 203, a smaller second safe operating distance 507 can be used.

In one embodiment, it takes approximately 1 second for target 112 topass through the entire second safe operating distance 507. The averagevelocity of target 112 during second coarse pass 514 therefore isapproximately 200 microns per second or 1200 microns per second,depending upon second safe operating distance 507.

While traversing second safe operating distance 507, target 112encounters a focused condition at second trip position 506. Whenelectronic focus signal 115 exceeds threshold value 304, flip flop 402latches and target 112 is stopped in the same manner as in first coarsepass 513. Target overshoot results in target 112 coming to rest belowfirst trip position 504. In FIG. 6, target 112 stops at second stoppingposition 508 and a microscope system 100 estimates the location of focusposition 203 based on overshoot and width of focus.

One or more additional passes can be used to make a better estimate(than the previous estimate) of focus position 203, depending on theresolution of ADC 407.

The steps performed during third coarse pass 515 are also similar oridentical to the steps described above for second coarse pass 514 andfirst coarse pass 513. Host work station 116, through microprocessor403, instructs coarse Z-stage controller 117 to move target 112 inpositive Z direction 533 at a velocity slower than the velocity oftarget 112 during second coarse pass 514. This motion continues until anear-focused condition 509 causes flip flop 402 to latch or until target112 moves through a predetermined third safe operating distance 510 to athird safe operating position 524. Again, third safe operating distance510 is selected to assure that target 112 travels through focus position203 and is dependent on the amount of target overshoot associated withsecond coarse pass 514.

In one embodiment, the third safe operating distance 510 is 140 microns.For target 112 to take approximately one second to pass through theentire third safe operating distance 510, the average velocity of target112 during the third coarse pass 515 is approximately 140 microns persecond in one embodiment.

Prior to traversing the entire third safe operating distance 510, target112 again encounters a near-focused condition at first trip position504. When the electronic focus signal 115 exceeds threshold value 304,flip flop 402, having been reset, latches and target 112 is stopped inthe same manner as in coarse passes 513 and 514.

Because of the relatively low velocity of coarse Z-stage 122 duringthird coarse pass 515, target 112 comes to rest at a third stoppingposition 511 that can either be above or below focus position 203. Afterthird coarse pass 515, the third stopping position 511 of target 112 iswithin approximately ±10 μm of focus position 203. To achieve thisdegree of accuracy, all of the factors contributing to target overshootare considered to empirically determine the amount of time required tostop target 112 upon encountering a focused condition during the thirdcoarse pass 515. Given the time required to stop target 112, thevelocity of target 112 during third coarse pass 515 is selected toassure that third stopping position 511 of target 112 is withinapproximately ±10 μm of focus position 203.

If full width half max measurement 305 of electronic focus signal 115 issufficiently wide, the coarse auto-focus operation described above ismodified. Although there is no clear boundary that determines when anelectronic focus signal 115 is "sufficiently wide", an electronic focussignal 115 created with a laser beam 123 having a wavelength of 488 nmand an objective lens 110 having a numerical aperture of 0.13 or loweris considered "wide," and has full width half max measurement 305 ofapproximately 29 microns. Electronic focus signal 115 exhibits a widerfull width half max measurement 305 as the magnification and numericalaperture of objective lens 110 decreases or as the wavelength of laserbeam 123 increases. For example, use of a low power (e.g. 1.5x or 5x)objective lens results in a large depth of focus, as illustrated byelectronic focus signal 115C (FIG. 3C).

When attempting a first coarse pass 513 on a "wide" electronic focussignal 115, first stopping position 505 of target 112 is relativelyclose to the second trip position 506. In certain cases, first stoppingposition 505 of target 112 is at a position where the value ofelectronic focus signal 115 exceeds the threshold value 304. That is,there is not enough target overshoot to guarantee that target 112"escapes" the electronic focus signal 115. Target 112 may "escape" if asa result of first coarse pass 513, target 112 is close to second tripposition 506 and is unable to obtain a high velocity before flip-flop402 latches during second coarse pass 514. As a result, second stoppingposition 508 of target 112 can be well short of peak 301. This conditioncauses the coarse auto-focus operation described above to miss focusposition 203.

In one embodiment, to perform a coarse auto-focus operation on a "wide"electronic focus signal 115 (FIG. 3C), target 112 (FIG. 1) is moved in astart-up move by 2000 microns (rather than 1000 microns described above)below the target's initial position (not shown) to starting position 516(FIG. 6). If target 112 is within 2000 microns of the lowermost position(not shown) along the Z-axis, target 112 is moved to this lowermostposition. This starting position can provide target 112 with anadditional distance in which to accelerate before reaching first tripposition 504. This acceleration and starting position ensure that target112 overshoots second trip position 506. The rest of the characteristicsof first coarse pass 513 for wide electronic focus signal 115C areidentical to those described above.

During second coarse pass 514 for wide electronic focus signal 115C,target 112 is moved downward along the Z-axis through second safeoperating distance 507. The second safe operating distance 507 and thevelocity of target 112 during the second coarse pass 514 are determinedin the manner previously described (i.e. based on a first estimatedetermined by the threshold method described above). However, ratherthan monitoring the status of flip-flop 402 during second coarse pass514, host work station 116 instructs microprocessor 403 to record thevalues of the electronic focus signal output by ADC 407 repeatedly at apredetermined interval (e.g. 80 times a second) while coarse Z-stage 122is moving target 112 at an approximately constant velocity.

The values recorded by microprocessor 403 therefore roughly correspondto magnitude of electronic focus signal 115 at regular distances alongthe Z-axis for the entire predetermined second safe operating distance507. Based on these recorded values, focus position 203 is calculated bya median point method described below. Because movement of coarseZ-stage 122 generally induces noise in the electronic focus signal 115,the values recorded by microprocessor 403 can exhibit peaks at positionsother than focus position 203 (as illustrated by FIG. 3C).Microprocessor 403 therefore filters the noise to obtain a betterestimate of focus position 203 in the "median point" method.

In the median point method host workstation 116 calculates the sum ofthe values previously recorded by microprocessor 403 and determines theelevation along the Z-axis at which half of this sum was reached duringsecond coarse pass 514. Host workstation 116 then issues a command tomove target 112 in the positive Z-direction to this position. The medianpoint method assumes that noise in electronic focus signal 115 is due toirregularity of movement of coarse Z-stage 122 and that thisirregularity occurs randomly and is equally probable on either side ofthe focus position 203 along the Z-axis.

In another embodiment of a microscope system 100 that uses a low powerobjective lens, during first coarse pass 513, instead of monitoring thestatus of flip-flop 402 according to the threshold method,microprocessor 403 records the values of electronic focus signal 115C(FIG. 3C) in the manner described above for second coarse pass 514 foruse in the median point method. Therefore, during first coarse pass 513,coarse Z-stage 122 (FIG. 1) moves target 112 through a predeterminedfirst safe operating distance 512 that is a safe travel limit of themovement of target 112. In this embodiment, starting position 516 is apredetermined distance of 1000 microns below the upper travel limit setby first safe operating position 520. Such a starting position 516 thatis independent of the target's initial position avoids the inherentuncertainty in the location of the focus position relative to startingposition if the target is moved by a predetermined distance from thetarget's initial position. At the end of first coarse pass 513, hostwork station 116 calculates a first estimate of focus position 203 byusing the median point method described above.

Then host workstation 116 determines starting and stopping positions ofsecond coarse pass 514. Starting position 531 is at a distance D (of 120microns in one embodiment), above first estimate of focus position 203while stopping position 522 is at the distance D after first estimatedfocus position 203. Distance D is chosen based on the desired precisionand the accuracy of first estimate of focus position 203. If 80 valuesare collected during second coarse pass 514, a 240 micron range ofmovement yields a resolution of 4 microns.

In this embodiment, host workstation 116 uses values recorded duringfirst coarse pass 513, to calculate (as described below) and set anauto-focus gain (e.g. gain of a PMT), so that, the intensity of theelectronic focus signal during second coarse pass 514 is optimal. Afterthe auto-focus gain is set, host workstation 116 instructs coarse Zstage 122 to move to second coarse pass starting position 531. Aftercoarse stage 122 has moved to starting position 531, host workstation116 instructs microprocessor 403 to start recording the values ofelectronic focus signal 115, and instructs coarse Z stage 122 to move tostopping position 522.

The calculation for optimal auto-focus gain depends on the specificsensor, and in one embodiment is as follows:

if "peak intensity" of the electronic focus signal is below optimal:then new auto-focus gain=old auto-focus ##EQU1## if the peak intensityis above optimal, then new auto-focus gain=old auto-focus gain-(peakintensity-195)/10,

where sensor gain is normalized to a range of 1 to 100, 1 being minimumgain, electronic focus signal intensity being normalized to the range of0 to 225, 0 being dark. The peak intensity (e.g. 195) of the electronicfocus signal is at maximum fraction (e.g. 3/4) of the maximum permittedintensity (e.g. 255), in one embodiment.

A maximum fraction (e.g. 3/4) is empirically chosen so that electronicfocus signal 115 is not too high (saturation) at focus position 203 andnot too low (indistinguishable from noise) at positions other than focusposition 203. Other maximum fractions, such as 2/3 or 4/5 can also bechosen, and the exact fraction that is chosen is not a critical aspectof this invention. If for some reason, the peak intensity of electronicfocus signal is less than a given minimum, the minimum is used in thecalculation. In one embodiment, 15 is the minimum intensity.

In one embodiment, first safe operating distance 512 is 1000 microns,yielding a 12 micron resolution for 80 samples of electronic focussignal 115. In this embodiment, second safe operating distance 537 is240 microns, that yields a 4 micron resolution for 80 samples ofelectronic focus signal 115.

Microfiche Appendix E illustrates one embodiment of software code thatimplements in host workstation 116 various computations for theauto-focus operations described above for use with a low power objectivelens. Routine lonui₋₋ StageAF in host workstation 116 initializesvariables and then invokes routine lon₋₋ APIQuery that determines thecharacteristics of objective lens 110. Then host workstation 116determines a travel limit fZLimUm and then finds out magnificationiObjPwr of objective lens 110. Then host workstation 116 instructs fineZ-stage 120 to move to a predetermined middle position HDWR₋₋ FASTZ₋₋MID₋₋ POS.

Host workstation 116 then saves values of several parameters currentlyin use, such as the intensity, system gain, zero value and Y axis scanamplitude. Then host workstation 116 sets a new zero value and a newsystem gain and invokes routine lonuiLoPwrStgAF for a low powerobjective lens or alternatively, performs various steps described belowfor a high power objective lens.

Routine lonuiLoPwrStgAF initializes variables, such as variabledHalfRange2ndPass and variable fZPos and then calls routinelonuiMoveStageZUm that moves target 112 to 1,000 microns below theuppermost travel limit set by first safe operating position 520 in oneembodiment to starting position 516 (FIG. 6). Then host workstation 116calls routine mot₋₋ SetCurrAxisSpeed that sets the target speed fortravelling for one second. Next, host workstation 116 calls routinelonuiMoveStgAndReadInten to move coarse Z-stage 122 to a stoppingposition 522 and during this movement reads the intensity (i.e.magnitude) of electronic focus signal 115. RoutinelonuiMoveStgAndReadInten returns with an estimate of focus position 203and the peak intensity. Based on the peak focus signal intensity, hostworkstation 116 adjusts auto-focus gain for optimal focus signalintensity in the next pass. If the peak intensity is lower than optimal,auto-focus gain is increased, if peak intensity is higher than optimal,auto-focus gain is decreased.

Routine zr₋₋ SetTargetLaserIFrImgInten sets the auto-focus gain based onthe intensity. Then host workstation 116 reads the current position bycalling routine stg₋₋ ReadStageZUm.

After optimal auto-focus gain is set, host workstation 116 thendetermines target 112's position in second coarse pass 514 as variablefTarget. Variable fTarget is normally the Z position of target 1128 forthe first estimate of focus position 203. If, during first coarse pass513 electronic focus signal 115's intensity is all zero, or ifphotodetector 114 is overloaded, then variable fTarget is set to 400micron away from coarse Z stage 122's position. Since actual focusposition 203 can be at some distance away from the first estimate offocus position 203 the second coarse pass 514 starts at some distancebefore the first estimate and ends at some distance after the firstestimate.

The distance before and after the first estimate is chosen to be severaltimes the depth-of-focus of objective lens 110 (in this embodiment 120microns) and is contained in variable dHalfRange2ndPass. If after firstcoarse pass 513, coarse Z stage 122 is at a position greater thandistance dHalfRange2ndPass away from estimate of focus position 203,then host workstation 116 instructs coarse Z stage 122 to move to astarting position at a distance dHalfRange2ndPass before first estimateof focus position, and second coarse pass 514 starts from this position.Otherwise, second coarse 514 starts from wherever first coarse pass 513ends. The stopping position of second coarse pass 514 is calculated tobe two times dHalfRange2ndPass away from starting position 514.

Then host workstation 116 changes the speed so as to move through secondsafe operating distance 537 within one second, by using routine mot₋₋SetCurrAxisSpeed. Host workstation 116 then moves target 112 throughsecond safe operating distance 537 by calling routinelonuiMoveStgAndReadInten, and computes the second estimate of focusposition 203.

After computing host workstation 116 moves target 112 to the secondestimate by calling routine lonuiMoveStageZUm.

After one of the previously described coarse auto-focus operations iscompleted, an optional fine auto-focus operation can be performed, ifelectronic focus signal 115 has a sharp peak, as illustrated in FIG. 3A.The zero position offset 310 and photodetector gain illustrated in FIG.3B are not utilized during a fine auto-focus operation. During a fineauto-focus operation, target 112 is moved by a fine Z-stage 120 thatincludes a piezoelectric element 1130. Fine Z-stage 120 is described indetail later.

Before a fine auto-focus operation is performed, target 112 ispositioned by a coarse auto-focus operation (above) such that focusposition 203 is within the operating range of fine Z-stage 120 (FIG. 1).In one embodiment, fine Z-stage 120 has an operating range of 50 micronsalong the Z-axis. Consequently, by performing the coarse auto-focusoperation described above, in which target 112 is positioned within+/-10 microns of the focus position 203, target 112 is positioned suchthat focus position 203 is within the operating range of fine Z-stage120.

FIG. 7 is a graphic representation of one embodiment (henceforth "first"embodiment) of a fine auto-focus operation as encoded in routineAFocusServo on page 7 of microfiche appendix A. Although FIG. 7illustrates two fine passes, any number of passes other than two can beused in other embodiments. The vertical axis in FIG. 7 illustrates theposition of target 112 along the Z-axis. The horizontal axis in FIG. 7illustrates the magnitude of electronic focus signal 115.

Prior to a first fine pass 601, microprocessor 403 sends a zeroingsignal to position control register 408 (FIG. 4). This signal istransmitted through DAC 409, summing node 410 and amplifier 411 topiezoelectric element 1130 (FIG. 11) in fine Z-stage 120. Fine Z-stage120, which was positioned in the middle of its operating range duringthe coarse auto-focus operation, moves to the lowermost position 619(FIG. 7) of its operating range (in a startup pass similar to thatdescribed above for a coarse auto-focus operation) in response to thezeroing signal. Microprocessor 403 then instructs control register 406to transmit a reset signal to clear flip flop 402. This instructionallows flip flop 402 to detect when electronic focus signal 115 exceedsthreshold value 304.

To begin first fine pass 601, microprocessor 403 transmits a series ofsignals to position control register 408, thereby causing fine Z-stage120 to move target 112 in positive Z direction 631 at a relatively highvelocity. In one embodiment, this velocity is approximately 75 micronsper second.

While it is necessary to precisely control the movement of target 112during first fine pass 601, piezoelectric element 1130 (FIG. 11) of fineZ-stage 120 has a slightly non-linear position response to the voltagesupplied by amplifier 411 (FIG. 4). To correct for this non-linearcharacteristic, a proximity sensor 1135 (FIG. 11) in fine Z-stage 120produces an electrical feedback signal that is transmitted to summingnode 410 (FIG. 4) and subtracted from the output signal of DAC 409 tocreate an error signal. When integrator 420 receives any non-zero errorsignal, integrator 420 generates an output signal that forces the errorsignal to zero. In this manner, integrator 420 compensates for thenon-linear response of the piezoelectric element 1130, thereby allowingfor linear control of the target's position by piezoelectric element1130.

First fine pass 601 uses the threshold method so that before target 112reaches the top of the operating range 620 of fine Z-stage 120,electronic focus signal 115 exceeds the threshold value 304 at a firsttrip position 604, because, as described above, the coarse auto-focusoperation ensures that focus position 203 lies within the range of fineZ-stage 120 (FIG. 7). At this time, flip flop 402 latches, therebyenabling a bit in status register 405. Microprocessor 403, whichcontinuously monitors status register 405, detects that the change instatus of flip flop 402 and signals position control register 408 tostop movement of fine Z-stage 120, and thereby stop movement of target112. Target 112 comes to rest at a first stopping position 605 which caneither be above or below focus position 203.

Consequently, prior to performing a second fine pass 615, microprocessor403 instructs fine Z-stage 120 to reposition target 112 a predeterminedfixed distance 614 in negative Z direction 632 such that a secondstopping position 608 of target 112 is below focus position 203. Factorsthat must be considered when selecting distance 614 are similar oridentical to the factors described above regarding the distance requiredto stop target 112 during first coarse pass 513. In one embodiment,distance 614 is 3.6 microns.

After target 112 has been repositioned at second stopping position 608,a second fine pass 615 is performed by moving target 112 in positive Zdirection 631 at a relatively low velocity (e.g. 7.5 microns persecond). During second fine pass 615, microprocessor 403 monitors theoutput of ADC 407, rather than the status of flip flop 402. The outputof ADC 407 is a digital representation of electronic focus signal 115.In this manner, microprocessor 403 measures the value of electronicfocus signal 115 as target 112 moves along the Z-axis.

Second fire pass 615 uses a peak detection method in which softwarewithin microprocessor 403 maintains an updated record of the maximumvalue output by ADC 407 and the position of target 112 at this maximumoutput value. Upon detecting an increasing value of electronic focussignal 115, followed by a decreasing value of electronic focus signal115, microprocessor 403 instructs fine Z-stage 120 to stop the motion oftarget 112 at third stopping position 611. Third stopping position 611is located above focus position 203 because of target overshoot whichoccurs for reasons similar to those described above. Microprocessor 403then instructs fine Z-stage 120 to move target 112 in negative Zdirection 632 to position 612 where the maximum value output by ADC 407was detected.

In one embodiment, position 612 is typically within one tenth of amicron of focus position 203. This accuracy is determined by thevelocity of target 112 and the number of bits used in ADC 407. Theslower the velocity of target 112 during second fine pass 615 and thegreater the number of bits used in ADC 407, the closer position 612 willbe to focus position 203. The entire fine auto-focus operation iscompleted in approximately one second in one embodiment. The completiontime is dependent upon the location of focus position 203 within therange of fine Z-stage 120. Computer code used to perform an auto-focusoperation in accordance with one embodiment of fine auto-focus method isset forth in Appendix A. The computer code of Appendix A is written inneuron C language, which requires an ECHELON compiler, available fromEchelon Corp. of 4015 Miranda Ave., Palo Alto, Calif. 94304.

FIG. 8A is a graphic representation of another embodiment (henceforth"second" embodiment) of a fine auto-focus operation. Although FIG. 8Aillustrates two fine passes, a number of passes other than two can beused in other embodiments. The vertical axis in FIG. 8A illustrates theposition (e.g. elevation) of target 112 along the Z-axis. The horizontalaxis in FIG. 8A illustrates the magnitude (i.e. strength) of electronicfocus signal 115. In first fine pass 750, target 112 is moved to each ofpositions 701-732. In second fine pass 850, target 112 is moved to eachof positions 801-832.

Prior to first fine pass 750, microprocessor 403 sends a zeroing signalto position control register 408 (FIG. 4). This signal is transmittedthrough DAC 409, integrator 420, summing node 410 and amplifier 411 topiezoelectric element 1130 of fine Z-stage 120. Fine Z-stage 120, whichwas positioned in the middle of its operating range before a coarseauto-focus operation (e.g., Step No. 2048 in FIG. 8A), moves to thebottom of its operating range (i.e., Step No. 0 in FIG. 8A) in responseto the zeroing signal, as illustrated by FIG. 8B.

First fine pass 750 uses a peak detection method. During first fine pass750, target 112 is moved in the positive direction, e.g. upward throughthe full range of motion of fine Z-stage 120 (i.e., 50 microns). Asshown in FIG. 8A, this range is divided into 4096 steps. Other numbersof steps can be used in other embodiments. Microprocessor 403 canposition target 112 at any one of these steps by sending a digital wordto DAC 409 (FIG. 4). During first fine pass 750, microprocessor 403sequentially provides 32 digital words to DAC 409, causing fine Z-stage120 to sequentially move target 112 to each of 32 positions 701-732.Each of the 32 positions 701-732 are separated by 128 steps(approximately 1.56 microns in one embodiment). At each of the 32positions 701-732, the output voltage of ADC 407 (corresponding to theelectronic focus signal 115) is repeatedly measured and digitallyfiltered (low-pass) by microprocessor 403 to obtain a single value foreach of the 32 positions 701-732. Microprocessor 403 saves the peakvalue of the output voltage and the position at which the peak valueoccurred.

Because the 32 positions 701-732 of first fine pass 750 are spaced 1.56microns apart, and electronic focus signal 115 has a depth of focus 302of approximately 2.54 microns, a focus position is not missed. Arelative peak value of the electronic focus signal is found within the32 positions 701-732 as long as there is enough system gain todistinguish electronic focus signal 115 from background value 303 andenough system gain such that electronic focus signal 115 can be detectedby ADC 407. System gain is determined by the laser power, the gain ofphotodetector 114, and the reflectivity of the sample. The presentinvention therefore has an advantage over prior art auto-focusmicroscopes, which require a much larger system gain in order for anauto-focus operation to be performed.

The position 718 at which the peak value is detected during first finepass 750 (illustrated as Step No. 2304 in FIG. 8A) becomes the center ofthe second fine pass 850. Microprocessor 403 instructs fine Z-stage 120to move target 112 to 128 steps below position 718 (i.e., to Step No.2176). In other embodiments, a different number of steps can be used.

Next, microprocessor 403 sequentially provides 32 digital words to DAC409, causing fine Z-stage 120 to sequentially move target 112 upward toeach of 32 positions 801-832. Each of 32 positions 801-832 are separatedby 8 steps (approximately 0.0977 microns). The total distance of secondfine pass is 3.125 microns. At each of 32 positions 801-832, outputvoltage of ADC 407 (corresponding to electronic focus signal 115) isrepeatedly measured and digitally filtered (low-pass) by microprocessor403 to obtain a single value for each of 32 positions 801-832.Microprocessor 403 saves the peak value of the output voltage and theposition at which the peak value occurred.

In yet another embodiment (henceforth "third" embodiment), first finepass 750 is performed as described above for the second embodiment andsecond fine pass 850 is performed by positioning target 112 at 128 stepsabove position 718 sample value was detected during first fine pass 750(e.g., at Step No. 2432 in FIG. 8A and at position 852 in FIG. 8C).Microprocessor 403 then sequentially provides 32 digital words to DAC409, causing the fine Z-stage 120 to sequentially move target 112 in thesame direction (e.g. downward) through 32 positions 832-801 in FIG. 8A.(See also FIG. 8C.) Moving in the same direction in second fine pass 850avoids the large acceleration caused by direction reversal at position851 (FIG. 8B).

After passing through the entire range (3.125 micron in one embodiment)of second fine pass 850, microprocessor 403 instructs fine Z-stage 120to position target 112 at position 812 at which a peak value is detectedduring second fine pass 850 (illustrated as Step No. 2272 in FIG. 8A).At the end of second fine pass 850, target 112 is positioned adjacent toactual focus position 203, i.e., within half the length between each ofthe 32 positions of the second fine pass 850 (within at least 0.0488microns in one embodiment).

An advantage of a second fine pass (and additional fine passes) thattarget 112 is positioned quickly and reliably without relying on athreshold value. In one embodiment, fine Z-stage 120 positions target112 sixty-seven times (including sixty-four positions at whichmeasurements are taken and three positions at which measurements are nottaken) to perform the fine auto-focus operation in approximately 0.9seconds. Computer code used to perform one embodiment an auto-focusoperation for a fine pass is set forth in microfiche appendix A.

A preferred embodiment of fine Z-stage 120 is shown in FIGS. 9-11.Microfiche appendix D at pages 65 and 66 lists various parts used in theembodiment shown in FIGS. 9-11. Referring to FIG. 9, fine Z-stage 120includes a square bottom plate 1010, four stationary bars 1020 fixedlyconnected along the edges of the bottom plate 1010, four rotating bars1030 pivotally connected to the stationary bars 1020 such that eachrotating bar 1030 is connected to one stationary bar 1020, and a squaretop plate 1050 pivotally connected along its edges to the four rotatingbars 1030.

A first set of horizontally-disposed flexures 1040 is connected betweenupper surfaces of the stationary bars 1020 and the rotating bars 1030,and a second set of vertically-disposed flexures 1060 is connectedbetween side surfaces of the rotating bars 1030 and the edges of the topplate 1050. In addition, the fine Z-stage 120 includes a piezoelectricactuator mechanism 1100 disposed in a space formed between the top plate1050 and the bottom plate 1010. Finally, an optional biasing spring 1140is connected between the top plate 1150 and the bottom plate 1110 forbiasing the top plate 1150 toward the bottom plate 1110.

Referring to FIG. 10, the bottom plate 1010 is preferably a flataluminum sheet 8 by 8 inches wide and 0.375 inch thick. As shown in FIG.10, the bottom plate 1010 includes a receiving hole 1011 within which islocated a pin 1012 for securing a first end of the optional biasingspring 1140. The bottom plate 1010 also includes an upper surface 1013.

Referring back to FIG. 9, the stationary bars 1020 are preferablyaluminum bars 5.5 inches long, 0.65 inch high and 0.5 inch wide. Thestationary bars 1020 are connected to the upper surface 1013 of thebottom plate 1010 using screws. The stationary bars 1020 are formed intoa square frame and located along the outer edges of the bottom plate1020. Referring again to FIG. 10, each stationary bar 1020 includes anupper surface 1021. A lip 1022 is formed along an outer edge of theupper surface 1021 of each stationary bar 1020.

The rotating bars 1030 are preferably aluminum bars which are 5.5 incheslong, 0.5 inch high and 0.6 inch wide. The rotating bars 1030 arepivotally connected to the stationary bars 1020, each rotating bar 1030being connected to one stationary bar 1020. Each rotating bar 1030includes an upper surface 1031 and an inner side surface 1032. A lip1033 is formed along a lower edge of the inner side surface 1032 of eachrotating bar 1030.

As shown in FIG. 10, thin horizontally-disposed flexures 1040 areconnected between the stationary bars 1020 and the rotating bars 1030.Each flexure 1040 is a thin sheet of 303 stainless steel which is 1 inchlong, 1 inch wide and approximately 0.01 inch thick. Each flexure 1040has one portion connected to the upper surface 1021 of a stationary bar1020 by a first fixture 1041, a second portion connected to the uppersurface 1031 of a rotating bar 1030 by a second fixture 1042, and asmall pivot portion 1043 located between the stationary bars 1020 andthe rotating bars 1030. Two flexures 1040 are connected between eachstationary bar 1020 and its associated rotating bar 1030, therebyrestricting each rotating bar 1030 to pivot around the pivot portion1043 such that the rotating bar 1030 remains in a parallel relationshipwith its associated stationary bar 1020.

The top plate 1050 is preferably a flat aluminum sheet 5.5 by 5.5 incheswide and 0.5 inch thick. Referring to FIG. 10, the top plate 1050includes a receiving hole 1051 within which is located a pin 1052 forsecuring a second end of the optional biasing spring 1140 (discussedbelow). In addition, the top plate 1050 defines threaded holes 1053(only one shown) for receiving preload screws 1054 and 1055 (see FIG.9). Finally, the top plate 1050 includes side surfaces 1056.

As shown in FIG. 10, thin vertically-disposed flexures 1060 areconnected between the top plate 1050 and the rotating bars 1030. Similarto the above-described horizontally-disposed flexures 1040, eachvertically-disposed flexure 1060 is a thin sheet of 303 stainless steelwhich is 1 inch long, 1 inch wide and 0.01 inch thick. Eachvertically-disposed flexure 1060 has one portion connected to the innerside surface 1032 of a rotating bar 1030 by a fixture 1061, a secondportion connected to a side surface 1056 of top plate 1050 by a fixture1062, and a pivot portion 1063 located between the rotating bars 1030and the top plate 1050. Two flexures 1060 are connected between eachrotating bar 1030 and one edge of the top plate 1050, therebyrestricting the top plate 1050 to pivot with respect to the rotatingbars 1030 such that the top plate 1050 remains in a parallelrelationship with the rotating bars 1030.

Referring to FIGS. 10 and 11, the piezoelectric actuator mechanism 1100includes a retainer block 1110 connected to the bottom plate 1010, arotating block 1120 integrally and pivotally connected to the retainerblock 1110, and an extending portion 1114 integrally and pivotallyconnected to the retainer block 1110. Further, a piezoelectric element1130 is received in the retainer block 1110 and has a free endcontacting the rotating block 1120. Finally, a sensor 1135 is receivedin the retainer block 1110 and generates a signal corresponding to anamount of rotation of the extending portion 1114.

Referring to FIG. 11, the retainer block 1110 is formed from 7075-T6high-strength aluminum alloy and is approximately 1.1 inch long, 0.6inch wide and 0.1 inch thick. The retainer block 1110 defines a firstthrough-hole 1111 for receiving the piezoelectric element 1130. Astainless steel plate 1112 is connected by fasteners 1113 to retain thepiezoelectric element 1130 within the through-hole 1111. In addition,the retainer block 1110 defines a second through-hole 1117 for receivingthe sensor 1135. The retainer block 110 also includes a hole 1118 forreceiving a sensor lock screw 1119 which contacts and secures the sensor1135.

As shown in FIG. 11, an extended portion 1114 is formed from 7075-T6high-strength aluminum alloy and is integrally connected to the uppersurface of the retainer block 1110 by a thin flexure. The extendedportion 1114 includes a socket 1115 for receiving a 0.125 inch diametersteel ball 1116 which contacts the top plate 1150. The extended portion1114 is preloaded downward by the preload screw pressing against thesteel ball 1116 (see FIG. 9). The sensor 1135 transmits a signalrepresenting a distance between the sensor and a side wall of theextended portion 1114 which varies in response to the rotation of theextended portion 1114. The signal is used to determine the verticaldisplacement of the top plate 1050, as discussed above.

As shown in FIGS. 10 and 11, the rotating block 1120 is a prism formedfrom 7075-T6 high-strength aluminum alloy and is approximately 1 inchlong. As shown in FIG. 10, the rotating block 1120 includes a verticalside wall 1121, an upper wall 1122 and a diagonal wall 1123. Therotating block 1120 is integrally connected to the upper surface of theretainer block 1110 by a flexure 1124. The side wall 1121 defines asocket 1125 for receiving a 0.125 inch diameter steel ball 1126 whichcontacts an end of the piezoelectric element 1130. In addition, theupper wall 1122 defines a second socket 1127 for receiving another 0.125inch diameter steel ball 1128 which contacts the preload screw 1154mounted in the top plate 1050. It is noted that the steel ball 1128 islocated further from the flexure 1124 than the steel ball 1116, which ismounted on the extended portion 1114.

The piezoelectric element 1130 is a cylindrical unit housed in thethrough-hole 1111 of the retainer block 1110 such that movement of afirst end is prevented by the plate 1112, and a second end contacts thevertical side wall 1121 of the rotating block 1120 through the ball1126. The piezoelectric element 1130 is connected to amplifier 411, asdiscussed above. A preferred piezoelectric element is sold by PhysicInstrument of Waldbronn, Germany under model number P830.20.

The sensor 1135 is also a cylindrical unit housed in the through-hole1117 and spaced a predetermined distance from the vertical side wall ofthe extended portion 1114. The sensor 1135 is connected to summing node410, as discussed above. Once the sensor 1135 is mounted a predetermineddistance from the side wall of the extended portion 1114, the sensorlock screw 1118 is tightened against the side of the sensor to preventmovement of the sensor 1135 within the through-hole 1117. A preferredsensor is sold by Kaman Instrumentation of Colorado Springs, Colo. undermodel number SMU 9000-15N.

Finally, an optional biasing spring 1140 may be connected between thepin 1012 formed in the bottom plate 1010 and the pin 1052 connected tothe top plate 1050.

In operation, when no actuating voltage is applied to the piezoelectricelement 1130, the optional biasing spring 1140 pulls the top platetoward the bottom plate until the top plate abuts and rests against theball 1116. In this position, the upper surface 1021 of the stationarybars 1020 and the upper surface 1031 of the rotating bars 1030 arealigned such that the horizontal flexures 1040 are substantially planar.In addition, the inner side surfaces 1032 of the rotating bars 1030 andthe side surfaces 1055 of the top plate 1050 are aligned such that thevertical flexures 1060 are substantially planar.

Upon application of an actuating voltage across the piezoelectricelement 1130, the piezoelectric element 1130 presses against the sidewall 1121 of the rotating block 1120 through the ball 1126, therebycausing the rotating block 1120 to rotate about a pivot portion 1062 ofthe flexure 1060 connecting the rotating block 1120 to the retainerblock 1110. As the rotating block 1120 is rotated away from the retainerblock 1110, the ball 1128 presses upward on the preload screw 1154,causing the top plate 1050 to move upward.

Upward movement of the top plate 1050 causes a rotation of the rotatingbars 1030. As the rotating bars 1030 are rotated, the horizontallydisposed flexures 1040 restrain the rotating bars 1030 such that theyremain parallel with their associated stationary bars 1020. In addition,the vertically disposed flexures 1060 cause the top plate 1050 to remainparallel with each of the rotating bars 1030. As a result, the rotatingbars 1030 act as torsion bars which prevent unwanted rotation ortranslation of the top plate 1050, there by resulting in the uppersurface of the top plate 1050 remaining parallel with the bottom plate1010.

Returning to FIG. 1, the mirror control 124 of FIG. 1 is used to rotateX-mirror 106 and Y-mirror 108 such that laser beam 123 can scan morethan a single point on target 112 while performing the coarse and/orfine auto-focus operations. Thus, if X-mirror 106 is rotated whileY-mirror 108 is held still, laser beam 123 will trace a line along theX-axis on target 112. Similarly, X-mirror 106 can be held still whileY-mirror 108 is rotated, thereby tracing a line along the Y-axis ontarget 112.

In one embodiment of the present invention, the coarse and fineauto-focus operations are performed by scanning a line, rather than aspot, on target 112. In this embodiment, the line scan is performed at afrequency of approximately 8 Khz while target 112 is held stationary atone of several elevations in the range of movement along the Z-axis.

For an area scan method, X-mirror 106 and Y-mirror 108 can both berotated to trace either a small area or selected parts of a larger areain the X-Y plane of target 112. The small area can be an area in thecenter of the field of view, while the larger area can be the entirefield of view. During the area scan, microscope system 100 coversvarious features in the field of view in a path similar to a raster pathof a television tube. To generate an electronic focus signal at acurrent Z-axis elevation, an area peak detector 1210 records and amicroprocessor 403 reads the value generated by the highest amount ofreflected light 123R that occurs during the area scan.

By using a line scan method or an area scan method and averaging theresult of the scan to create electronic focus signal 115, the estimateof focus position 203 during a coarse auto-focus operation or a fineauto-focus operation becomes less sensitive to local height variationson the surface of target 112.

In a preferred embodiment, the area scan method is used for a fineauto-focus operation, although an area scan can also be used for acoarse auto-focus operation. In this embodiment, area peak detector 1210(FIG. 12) is synchronously reset at the start of the area scan (anywhereon the area can be used as the start, as long as the reset issynchronous). FIG. 12 is similar or identical to FIG. 4, except forcertain components that are described below. Area peak detector 1210acquires or "loads up" during the rest of the area scan as follows. Areapeak detector 1210 has a storage capacitor C67 (FIG. 14) that isdischarged when area peak detector 1210 is reset. The voltage on storagecapacitor C67 is continuously compared to the input voltage of op-ampU26. When op-amp U26's input voltage exceeds the stored voltage ofcapacitor C67, capacitor C67 is charged up to equal the input voltage.This process happens continuously, so that storage capacitor C67 recordsthe highest input voltage to op-amp U26 no matter how short itsduration, (within bandwidth limitations) that occurs after the reset.The only way that storage capacitor C67 discharges is through the resetmechanism or through leakage currents.

Just before being reset for the next area scan, a sampling ADC 5021digitizes the output of area peak detector 1210. The output of ADC 5021represents electronic focus signal 115 for the current elevation oftarget 112. ADC 5021 has a track-and-hold buffer in front (in oneembodiment buffer is part of the architecture of ADC 5021), so thatacquiring the output of area peak detector 1210 is virtuallyinstantaneous. The ADC can digitize a voltage at the output of area peakdetector 1210 at 0.386 V/μs without error, that translates to anacquisition time of about 25 ns. Area peak detector 1210 can be resetvery quickly, using for example, a 500 nanosecond pulse. The time duringwhich output of area peak detector 1210 is digitized and area peakdetector 1210 reset is an extremely small portion of the overall areascan time so that only a very small portion of the area (e.g. 0.15%) isignored while area peak detector 1210 is reset.

Normally an X-axis laser scanner (not shown) (also referred to as "linescanner") that is used in microscope system 100 is resonant, while aY-axis scanner (not shown), is a closed-loop galvo scanner that followsa "sawtooth" waveform (relatively slow scan followed by a fast flyback)during normal imaging that is unrelated to an auto-focus operation.

For an area scan used to find focus position 203, the Y-axis scanner(also referred to as "page scanner") follows a triangular wave profile1301 (FIG. 13A) that is different from the. "sawtooth" waveform at amuch higher scan rate (e.g. 125 Hz) than normal (e.g. 13 Hz). The highscan rate allows microscope system 100 to trace a raster (e.g. sinewave) path in the area in the field of view quickly (8 msec in oneembodiment), albeit at less resolution (e.g. 32 lines/frame). At the endof an auto-focus operation, once target 112 has been positioned at focusposition 203, the page scanner is returned to its normal slow sawtoothwaveform.

Buffer 1230 connected to the output terminals of ADC 5021 allows signalson ADC 5021's output terminals to be turned on and held on withoutinterfering with the operation of data bus 5006. This is requiredbecause ADC 5021 used in this embodiment cannot be configured to do aconversion without turning on signals at its output terminals.Conversion is initiated by hardware at a very precise predeterminedtime, at the end of an area scan. Signals that result from conversionremain active at the output terminals of ADC 5021 until microprocessor403 detects that a scan is complete, reads ADC 5021 through buffer 1230by asserting signal R₋₋ ADC* and resets ADC 5021 by pulsing signalRST-ZSYNC*.

FIG. 13B illustrates events after the rising edge of signal ZSYNC.Signal ZSYNC rises at the end of each page scan to indicate completionof a scan cycle. The time scale in FIG. 13B is different from that ofFIG. 13A. Signal ZSYNCD*, that is a saved version of signal ZSYNC, goeslow in response to a rising edge in signal ZSYNC. At the same time,signal CNVRT* (FIG. 13B) also goes low, initiating an analog to digitalconversion and causing signal ADC₋₋ BSY* (FIGS. 13B) to go low. SignalBUSY1* and signal BUSY2* are delayed versions of signal ADC₋₋ BSY*, andare used to generate an active high pulse in signal RST₋₋ PKDET (FIG.13B) after signal ADC₋₋ BSY* returns high, to reset peak detector 1220after ADC's conversion is completed. So ADC 5021 automatically starts aconversion when signal ZSYNC goes high, and area peak detector 1210 isautomatically reset when the conversion is complete. Because there is aT/H in ADC 5021, the peak detector 1220 can be reset after the T/H hasacquired, and before the conversion is complete.

Signal CNVRT* stays low until signal RST₋₋ ZSYNC* falls, which keeps theconversion result present at the output terminals of ADC 5021. Softwarein microprocessor 5000 reads ADC 5021 (via signal R₋₋ ADC*) beforeresetting signal ZSYNCD*. Signal ZSYNCD* is an input signal tomicroprocessor 5000 and indicates that a frame is complete.

In one embodiment, there is a relatively long and unpredictable latencyin the response of host workstation 116 to signal ZSYNCD*. Hence most ofthe control signals (e.g. signals RST₋₋ PKDET and CNVRT*) for area peakdetector 1210 and ADC 5021 are generated in hardware.

Area peak detector 1210 consists primarily of op-amps U26 and U27 (FIG.14). The "hold" capacitor for area peak detector 1210 is C67. When theinput voltage at pin 3 of op-amp U26 is greater than the hold voltage atcapacitor C67, D9 is reverse-biased, so op-amp U26 has no feedback (atleast momentarily). The output of op-amp U26 will therefore rise untilop-amp U27 responds to the increased output of op-amp U26 and providesfeedback via resistor R47. In this manner, when the input voltage toop-amp U26 is greater than the hold voltage, op-amp U26 drives the holdvoltage higher to equal the input voltage. When the input voltage toop-amp U26 is less than the hold voltage, a diode D12 is reverse-biasedand op-amp U26 receives a feedback signal through diode D9. Becausediode D12 is reverse-biased, changes in the input voltage at op-amp U26do not affect the hold voltage when op-amp U26's input voltage is lessthan the hold voltage.

Op-amp U27 buffers the hold voltage to minimize the current drawn fromhold capacitor C67 (and hence the droop rate), and to provide feedbackto op-amp U26. Transistor Q2 resets hold capacitor 67. Transistor Q1guarantees that the input voltage of op-amp U26 is less than the holdvoltage during reset (the hold voltage is zero during reset), so thattransistor U26 does not slew positive during the reset operation. Whenthe input voltage at U26 pin 3 is greater than the hold voltage at C67,D9 is reverse biased, so U26 has no feedback (at least momentarily).U26's output rises (slews positive) at the maximum rate possible for theparticular op-amp (its "slow side") until U27 responds to the increasedoutput of U26 and provides feedback via R47.

Diode D10 and resistor R46 establish a small negative voltage (approx.-0.4 V in one embodiment) for clamping the input signal during reset.Resistors R53 and R47 slow down area peak detector 1210, to reduceovershoot that is common in conventional area peak detectors. ResistorR53 limits the rate at which capacitor C67 can be charged, resulting inan acquisition bandwidth of 2.3 MHz in one embodiment. Resistor R47works with parasitic capacitance of U26 and D9 to form a feedback filterfor composite op-amp U26/U27. Resistor R47 acts to stabilize (reduceovershoot) of the composite op-amp. Both resistors R53 and R47 aredetermined empirically. Even with resistors R53 and R46, area peakdetector 1210 provides an extremely fast response, and the combinationof resistor R52 and capacitor C64 acts as a filter to limit thebandwidth of the input signal, so that high frequency noise does notpass through to area peak detector 1210.

An auto-focus routine AFPeakSync (page 37 of microfiche appendix B) thatperforms an auto-focus operation during using an area scan method by amicroscope system 100 is similar to auto-focus routine AFFast (page 36of microfiche appendix B) described above (FIG. 8B). As seen in FIG. 8B,in a first fine pass, auto-focus routine AFFast steps from bottom to topof the fine Z-axis range of movement in large steps, then returns to aposition below the coarse step that had the highest focus signal.

In a second fine pass, auto-focus routine AFFast then steps upwardagain, but this time in small steps (e.g. 0.098 μm). Auto-focus routineAFFast causes a large acceleration due to direction reversal just beforethe second fine pass by stepping to a position just below the estimatedfocus position (FIG. 8B). Larger accelerations imply larger positionerrors in fine Z-stage 120 and are preferably avoided to improvepositioning fidelity.!

Auto-focus routine AFPeakSync also uses two fine passes: a first finepass and a second fine pass. The first fine pass of routine AFPeakSync(henceforth "AFPeakSync first fine pass") differs from the coarse passof the coarse Z-stage in the manner described below. The AFPeakSyncfirst fine pass operates similar to the coarse pass of auto-focusroutine AFFast. There is no synchronization, and auto-focus routineAFPeakSync steps and measures as quickly as possible, using large steps(e.g. 1.56 micrometer).

Software (Appendix F) in host workstation 116 sets up the page scanner(not shown) to perform the area scan method during the AFPeakSync firstfine pass. Routine lonuiSuperfineAF sets up all parameters for use inthe area scan method. In routine lonuiSuperFineAF, function lonui₋₋Get₋₋ IndexFrPixZoomEnum returns with the current X and Y scannerparameters, such as amplitude of oscillation. After the scannerparameters are set, host workstation 116 instructs fine Z-axiscontroller 118 to execute a fine auto-focus operation. After the fineauto-focus operation is done, host workstation 116 returns scannerparameters to their original values.

In alternative embodiments, the page scanner can execute a line scanmethod or a spot method during the AFPeakSync first fine pass. TheAFPeakSync first fine pass is merely used to move target 112 closeenough to focus position 112 so that a AFPeakSync second fine pass cancorrect residual errors.

Auto-focus routine AFPeakSync avoids the large acceleration ofauto-focus routine AFFast in a second fine pass by stepping to aposition above the estimated focus position at the end of a first finepass and continuing in small steps for the second fine pass in the samedirection, as illustrated by FIG. 8C.

In one embodiment, a second fine pass in auto-focus routine AFPeakSynccovers twice the range of a fine pass in auto-focus routine AFFast,which reduces sensitivity to manufacturing variations and improvesperformance of the autofocus operation. The increased range of theAFPeakSync second fine pass compensates for any position errors in theAFPeakSync first fine pass that could result in a AFPeakSync step withthe highest focus strength being off by one rough step.

Auto-focus routine AFPeakSync spends two page scanner cycles at eachposition of the AFPeakSync second fine pass. The first cycle allows thefine Z-axis (signal ACTUAL Z POSITION in FIG. 13C) to settle out, andthe second cycle acquires the peak magnitude of electronic focus signal115 from the page scan at the current elevation. Another embodiment ofan auto-focus routine can spend only one page scanner cycle by allowingthe fine Z movement to settle during a trace 1310 and by acquiring thepeak during the retrace 1320.

Auto-focus routine AFPeakSync executes in about 1.5 seconds, compared to0.9 seconds for auto-focus routine AFFast because of the time requiredto perform an area scan at each of several elevations of target 112. Inspite of such an increased execution time, a microscope system that usesauto-focus routine AFPeaksync is faster overall as compared to amicroscope system that uses auto-focus routine AFFast, because withincreased reliability of the auto-focus operation using routineAFPeaksync, manual focus adjustments are needed only rarely.

The first fine pass of auto-focus routine AFPeakSync steps as quickly aspossible irrespective of where the scanner is during sampling. In thesecond fine pass, however, auto-focus routine AfPeakSync runs the pagescanner at a high rate (>100 Hz) and allows one page scanner cycle forfine Z-stage 120 to settle, before doing data acquisition on the nextcycle, as illustrated in FIG. 13C. Auto-focus routine AFPeakSynctherefore spends two page scanner cycles at each Z position (e.g.elevation). Furthermore, in area peak detector mode, ananalog-to-digital converter 5021 is automatically started when a risingedge in signal ZSYNC occurs, and area peak detector 1210 isautomatically reset. So, auto-focus routine AFPeakSync causes hostmicroprocessor 5000 to wait for a rising edge in signal ZSYNC and thenread the peak magnitude detected from the previous page scan fromanalog-to-digital converter 5021. Resetting signal ZSYNC also resetsanalog-to-digital converter 5021 and so ADC 5021 is read beforeresetting signal ZSYNC.

In peak detector mode, analog-to-digital converter 5021 is controlledonly by a rising edge in signal ZSYNC. An asynchronous conversion is notpossible without resetting the peak detector mode bit. Peak detector1210 obtains a representation of the highest intensity that occurred ina video frame at a given Z elevation. Peak detector 1210 allowsmicroscope system 100 to repeatably focus on the layer with the highestreflectivity.

Microscope system 100 can use programmable offset FocOff to allow theuser to select any predetermined layer to focus on. In one embodiment,microscope system 100 initially focuses on a layer ("brightest layer")of target 112 that generates the largest electronic focus signal. Theuser can adjust the position of target 112 to bring a predeterminedlayer into focus. Microscope system 100 records the target's adjustmentas an offset from the brightest layer. In subsequent auto-focusoperations, microscope system 100 focuses on the brightest layer andthen automatically moves target 112 through the adjustment offset thatwas specified by the user, to focus on the predetermined layer.

As the relative reflectivities of the layers in a wafer are repeatableand independent of the illumination power, the relative positions of apredetermined layer chosen by a user is at a repeatable offset from thebrightest layer. So microscope system 100 can repeatably focus on anypredetermined layer after a single adjustment. Microscope system 100focuses on such a predetermined layer as long as the layer structurestays constant, for example for array type structures, such as RAMarrays. If, after selecting a predetermined layer, areas off a RAM arrayare to be imaged, a simple re-adjustment allows the user to continue.

In one embodiment, the function to specify the offset is implemented asa system function that can be mapped by a user to any function key ofhost workstation 116. Once mapped to a function key, the offset functioncan be accessed by the user from the keyboard at any time. When soaccessed, host workstation 116 compares the current target position withthe focus position estimate of last auto-focus operation, calculates thedifference and stores the difference as the offset for finding thepredetermined layer in future auto-focus operations.

The offset is limited to +/-5 microns, to make it easier to recover froman improperly set offset (a typical semiconductor topology is less than1 μm) and only fine Z-stage 120 is moved during offset calculation. Theoffset is calculated assuming coarse Z-stage 122 has not moved sincelast auto-focus operation.

While the present invention has been described in connection withspecific embodiments, variations on these embodiments will be obvious tothose having ordinary skill in the art. For example, target 112 may bemoved by means other than a stepper motor or a piezoelectric element,such as a linear voice coil motor or an electrostrictive actuator.Furthermore, target 112 can initially be positioned above the focusposition, with the first pass in any of the embodiments beginning bymoving target 112 in a negative Z direction. In addition, although thepresent invention was described in connection with a microscope thatreflects a maximum intensity to the photodetector during a focusedcondition, it is clear that the invention may be modified to operatewith a microscope that reflects a minimum intensity to the photodetectorduring a focused condition.

Furthermore, although the invention, as described, utilizes a laser beam123 to perform both the auto-focusing and imaging operations, it isunderstood that a confocal laser optical system could be used to performan auto-focusing operation for a non-confocal microscope system whichutilizes only a white light source to perform the imaging operation.Such an application is advantageous because a non-confocal microscopesystem utilizing a white light imaging source results in a focus signalwhich is sinc function, rather than a sinc squared function. Because thesinc function does not exhibit a peak which is as pronounced as the sincsquared function, it is more difficult to determine the focus positionusing a focus signal generated by a non-confocal, white light opticalsystem. Therefore, using a confocal laser optical system to generate thefocus signal used to perform the auto-focus operation in a white lightmicroscope results in a superior electronic focus signal, therebyallowing the auto-focus operation to be performed with greaterprecision. Confocal white light can also be used, with scanning replacedby detector array, such as a camera.

Although a certain number and types of passes are described above, othernumber and types of passes can also be used. For example a microscopesystem 100 can perform a single pass in a coarse auto-focus operationusing a median point method and adjust the gain of photodetector 114followed by two passes in a fine auto-focus operation to estimate focusposition 203.

Various embodiments of the invention described above are encompassed bythe attached claims.

What is claimed is:
 1. A method comprising:(a) moving a target through adistance in a direction relative to a lens, the distance being definedbetween first and second target positions; (b) generating an electronicfocus signal during (a), the focus signal having a magnitude that is afunction of an intensity of light reflected from the target; (c)recording, during (a), a plurality of values corresponding to themagnitude of the electronic focus signal at a plurality of points alongthe distance.
 2. The method of claim 1, wherein the target is moved by astepper motor or by a piezo electrically driven element.
 3. The methodof claim 1, further comprising the step of setting up gain and adjustingthe gain if the target moves through the distance without the magnitudeof the electronic focus signal exceeding a predetermined optimal value.4. The method of claim 1, wherein the step of generating furthercomprises:measuring the intensity of light reflected from the targetthrough a pin hole in a direction reverse to the incident path of thelight; and transforming the measured intensity into the electronic focussignal.
 5. The method of claim 4, wherein the light is a laser beam, andwherein the step of transmitting further comprises moving the laser beamto define a line on the surface of the target.
 6. The method of claim 1,further comprising:(d) calculating an estimated focus position using theplurality of values.
 7. The method of claim 6, wherein (d) comprises:(e)summing the values to obtain a sum.
 8. The method of claim 7, furthercomprising:(f) dividing the first-mentioned sum by a number to obtain asecond sum less than the first sum.
 9. The method of claim 8, whereinthe number is two.
 10. The method of claim 8, further comprising:(f)determining a third target position along the distance at which a thirdsum of a subset of the plurality of values defined between the firsttarget position and the second target position exceeds the second sum.11. The method of claim 6, further comprising:(e) moving the targetthrough a second distance relative to the lens, the second distancebeing defined between third and fourth target positions; (f) generatingthe electronic focus signal during (e); and (g) recording, during (e), asecond plurality of values corresponding to the magnitude of theelectronic focus signal at a plurality of points along the seconddistance.
 12. The method of claim 11, wherein (e) includes moving thetarget relative to the lens in a second direction opposite thefirst-mentioned direction.
 13. The method of claim 11, wherein the fifthtarget position is a stop position derived from the estimated focusposition.
 14. The method of claim 11, wherein the target is movedthrough the first distance at a first velocity and through the seconddistance at a second velocity, the second velocity being less than thefirst velocity.
 15. The method of claim 11, further comprising:(h)calculating a second estimated focus position using the second pluralityof values.
 16. The method of claim 15, further comprising:(i) summingthe second plurality of values to obtain a sum.
 17. The method of claim16, further comprising:(j) dividing the first-mentioned sum by a numberto obtain a second sum less than the first sum.
 18. The method of claim17, wherein the number is two.
 19. The method of claim 17, furthercomprising:(k) determining a fifth target position along the distance atwhich a third sum of a subset of the plurality of values defined betweenthe first target position and the second target position exceeds thesecond sum.
 20. An apparatus comprising:(a) means for moving a targetthrough a distance in a direction relative to a lens, the distance beingdefined between first and second target positions; (b) means forgenerating an electronic focus signal during (a), the focus signalhaving a magnitude that is a function of an intensity of light reflectedfrom the target; (c) means for recording, during (a), a plurality ofvalues corresponding to the magnitude of the electronic focus signal ata plurality of points along the distance.
 21. The apparatus of claim 20,wherein the means for generating further comprises a means for scanninga portion of the target with the light, the apparatus further comprisinga peak detector coupled to the means for recording, the peak detectordetecting a maximum value of the electronic focus signal encounteredwhile the light is scanned across the area.
 22. The apparatus of claim20, wherein the portion is an area of the target.